Cisco Systems UBR7225VXR specifications Troubleshooting Verifying the Downstream Signal

Chapter 6 Troubleshooting

Verifying the Downstream Signal

snr_estimate 352(TenthdB), ber_estimate 0, lock_threshold 26000 QAM in lock, FEC in lock, qam_mode QAM_64

This shows a downstream frequency of 555 MHz, 64-QAM downstream, and an SNR estimate of 35.2 dB.

Note Use the debug cable mac log verbose and show controllers cable 0 mac state commands when troubleshooting the cable interface’s initialization sequence and locking on the downstream signal.

Step 2 Scan the output for the value corresponding to the “SNR_estimate” variable. If this value is at least 35 dB, then you have an optimized signal. If the value is less than 34 dB, you must adjust the cable interface line card.

a.Verify that the line card DS output level is correct (+32 dBmV or +42 dBmV).

b.Make sure that the Cisco uBR7225VXR line card IF input to the upconverter is within an acceptable range (check the upconverter manufacturer’s specifications).

c.Check the upconverter RF output level. A typical setting is +55 to +58 dBmV.

d.Make sure that the upconverter RF output center frequency has been set correctly.

e.Make sure that the proper amount of attenuation has been installed between the upconverter RF output and the headend combiner input so that the digitally modulated carrier’s average power level is 6 dB to 10 dB below the level of the analog TV channels.

f.Make sure that the digitally modulated carrier’s RF average power level is in the –15 dBmV to +15 dBmV range at the input to the cable router.

Note The exact value displayed will vary from cable interface to cable interface; however, the values collected from measurement to measurement on the same router will be consistent.

Step 3 If the previous steps do not identify the problem, check the downstream digitally modulated carrier using a QAM analyzer. Several third-party test equipment manufacturers have these instruments available.

a.Measure modulation error ratio (MER), which should be 35 dB or greater in the headend.

b.Check pre- and post-FEC bit error rate (BER). There should be no bit errors in the headend. If bit errors are observed, one likely cause is sweep transmitter interference.

–Check the sweep transmitter to make sure that adequate guard bands have been programmed around the digitally modulated carrier.

–Bit errors also may be caused by clipping of the digitally modulated carrier in the upconverter or other active device, including amplifiers and optical transmitters.

c.Evaluate the digitally modulated carrier’s constellation. Look for evidence of:

–Gain compression

–Phase noise

–Inphase and quadrature (I-Q) imbalance

–Coherent interference

–Excessive noise

–Clipping

Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide

6-8 OL-17309-02